One of the hallmarks of the inflammatory reaction is the accumulation of leukocytes at sites of injury or infection. During an immune response, circulating blood monocytes traverse the endothelial monolayer and the basement membrane of a blood vessel to reach an inflammatory site. This transmigration requires a multistep process involving an interaction between leukocytes and endothelial cells. Once at the inflammatory site, the monocytes interact with a wide variety of substances that include cytokines and the components of the extracellular matrix (ECM), such as collagen. Monocytes are then stimulated to differentiate into macrophages or antigen-presenting dendritic cells (DCs).
DCs are pivotal antigen-presenting cells for initiation of an immune response. Indeed, dendritic cells provide the basis for the production of an effective immune response to a vaccine, particularly for antigens wherein conventional vaccination is inadequate. DCs are also important in the production of an immune response to tumor antigens. Components of the ECM, such as collagen, may be involved in regulating the maturation of DCs, and may alter the ability of DCs to present antigens to the other cells of the immune system.
Collagen, a major component of the ECM, is a ligand for the discoidin domain receptor (DDR) tyrosine kinases DDR1, and DDR2. DDRs are a subfamily of receptor tyrosine kinases that possess an extracellular domain related to the lectin discoidin, found in the slime mold Dictyostelium discoideum. All members of the subfamily share an approximately 160-amino acid-long amino terminal discoidin (DS) homology domain, a single transmembrane region, extended juxtamembrane region, and a catalytic tyrosine kinase domain. DDR1, appears in at least five isoforms, a, b, c, d, and e, which are generated by alternative splicing. The DDR1b isoform contains 37, additional amino acids in the juxtamembrane region, relative to the DDR1a isoform. Contained within the additional amino acids in DDR1b is the LXNPXY motif that corresponds to the consensus binding motif for the Shc phosphotyrosine binding (PTB) domain. Collagen activation of DDR1b induces the autophosphorylation of DDR1b at specific residues within the binding motif.
Although the biological function of DDR1, is unknown, its expression pattern has been analyzed in a variety of normal and malignant tissues. Northern blot and in situ hybridization analysis demonstrate that human DDR1, is expressed predominantly in epithelial cells, such as those in the kidney, lung, gastrointestinal tract and brain. Upregulated DDR1, expression has been detected in breast, ovarian, esophageal, and brain tumors, and has been demonstrated to be at least three-fold higher in tumor cells than in the adjacent, normal, epithelium. Thus, DDR1 activation may be involved in tumorigenesis.
DDR1, mutants and DDR1-null mice have been used to study the function of DDR1. DDR1-null mice lack DDR1, protein expression. These mice are born alive, but are small compared to their heterozygous littermates. The DDR1-null females show multiple reproductive defects, although the female DDR1-null mice that are successful at reproducing are unable to nourish their litters because the mammary gland epithelium fails to secrete milk.
Regulating the activation of DDR1, may be important for controlling cell growth and differentiation. Thus, there exists a need for identifying agents that regulate the activation of DDR1, and, in turn, regulate the activation of DDR1 downstream signaling molecules.